This application relates generally to the monitoring of gas pressure levels in an enclosed vessel and more particularly to apparatus for monitoring a pressurized gas used in hybrid air bag systems for motor vehicles and a method for making such apparatus.
The use of air bags for passengers, as well as drivers, in motor vehicles is becoming more and more widespread. Early versions of air bag systems made use of pyrotechnic inflators in which a rapid oxidation of sodium azide cause the air bags to inflate in a very short period of time; however, the use of sodium azide has several disadvantages. It is hazardous to process, toxic to unprotected workers and is a powerful, unstable explosive during processing.
As a result, a modified air bag inflator is becoming more popular. In the modified system, known as a hybrid inflator, a pressurized gas, typically argon, an inert gas, is used in combination with a smaller amount of less hazardous solid propellant to inflate the air bag. Although hybrid systems take slightly longer to effect inflation and require a heavier steel housing rather than aluminum used for the pyrotechnic versions, substituting argon gas for a solid propellant substantially lowers the cost of the inflation system and is less hazardous to process.
A module used in an air bag system comprises an inflator, a metal housing and an inflatable bag. The hybrid version also requires a separate electronic circuit including a sensor to monitor the bottle pressure for the stored gas. That is, it is necessary to be able to determine that the pressure of the stored gas is above a selected level to ensure effective deployment of the air bag when called for.
Space occupied by air bag systems currently in use is at a premium, particularly with respect to the drivers' side. As a result, the space available for the sensor system is very limited. In U.S. Pat. No. 5,331,126 assigned to the assignee of the present invention, a sensor system in the form of a pressure responsive switch useful in hybrid air bag systems is shown and described. The assembly comprises a pressure responsive disc movable between oppositely dished concave, convex configurations having one side exposed to gas contained in a small gas reference chamber. The opposite side of the disc is operatively connected to a movable contact arm of an electric switch and is exposed to the gas used for inflation which is stored in a container, also referred to as a bottle. The assembly is hermetically attached to a wall within the air bag bottle and when terminals of the switch are connected to a suitable voltage source the switch will monitor the pressure in the air bag bottle. If the pressure in the bottle should decrease to a selected level indicating inadequate inflator bottle pressure, the disc will snap to open a circuit.
Although different gases can be used in the air bottle, argon, as mentioned above, has become the usual choice due in part to its properties and availability, i.e., it is inert, relatively heavy and has a sufficiently low liquefaction temperature. A small amount of helium, usually in the range of 2-5% is added in order to facilitate leakage testing. Leakage of gas from the air bag bottle is checked at the time of manufacture to ensure that sufficient pressure will remain in the bottle for intended operation of the system over a selected life span, e.g., fifteen years and helium is used for this purpose since suitable equipment and techniques are available for detecting and measuring helium.
The air bag bottle is charged with the gas mixture at a selected pressure and temperature and the reference chamber is charged to a pressure slightly lower so that in the event that gas does leak from the bottle, when the pressure in the bottle decreases to a calibrated level, the differential pressure between the stored gas and the gas in the reference chamber will cause the disc to snap and actuate the electrical switch and provide a warning of the low pressure condition.
By way of example, a typical air bag bottle for the passenger side of a vehicle uses a mixture of 98% argon and 2% helium and is charged at room temperature to 2800 psi. The system is considered to provide effective operation as long as the pressure remains above 2400 psi at room temperature. In order to provide effective monitoring, when using a switch assembly of the type set forth in the above referenced patent, the reference chamber is charged to a pressure of approximately 2300 psi so that, along with the release pressure of approximately 100 psi of the disc, switching will occur at the desired level.
However, charging the reference gas to a lower pressure than the stored gas results in different slopes of the pressure vs temperature curves if the same gas mixture is employed. This causes a tracking problem since the system needs to be operable over a wide temperature range, i.e., from -40.degree. C. to 90.degree. C. Another problem occurs with regard to leakage testing to ensure that the device will meet specifications for the expected life. The reference chamber is so small and at 2% the total amount of helium is so little that it is impractical, if not technically impossible, to reliably test for such minute quantities with currently available equipment.